The invention relates to a heat sealing device useful for making pouches filled with flowable materials, e.g. liquids, on so-called vertical form, fill and seal machines. It is well known to package flowable materials, for example, milk, syrups, sauces, on so-called vertical form, fill and seal machines. Using such a machine, a flat web of synthetic thermoplastic film is unwound from a roll and formed into a continuous tube in a tube forming section, by sealing the longitudinal edges of the film together to form a so-called lap seal or a so-called fin seal. The tube thus formed is pulled vertically downwards to a filling station. The tube is then collapsed across a transverse cross-section of the tube, the position of the cross-section being at a sealing device below the filling station. A transverse heat seal is made, by the sealing device, at the collapsed portion of the tube, thus making an airtight seal across the tube. The sealing device generally comprises a pair of jaws. During and after making the transverse seal a quantity of material to be packaged, e.g. liquid, is caused to enter the tube, at the filling station, and fill the tube upwardly from the aforementioned transverse seal. The tube is then caused to move downwardly a predetermined distance. Such movement may be under the influence of the weight of the material in the tube, or may be caused by pulling or mechanically driving the tube. The jaws of the sealing device are closed again, thus collapsing the tube at a second transverse section. The second transverse section may be above, usually just above, the air/material interface in the tube, or the second transverse section may be below the air/material interface. The sealing device seals and severs the tube transversely at the second transverse section. The material-filled portion of the tube is now in the form of a pillow shaped pouch. Thus the sealing device has sealed the top of the filled pouch, sealed the bottom of the next-to-be formed pouch, all in one operation. One such vertical form and fill machine of the type described above is sold under the trade mark PREPAC. With some other machines, the sealing device does not sever the tube at the second transverse section, but does sever the tube subsequently.
It is sometimes desired to produce a "slack" pouch. A slack pouch is one that has little or no "headspace", i.e. has substantially no air in the pouch, and is not filled to capacity with product. For example, slack pouches filled with refrigerated product, e.g. taco meat, are more easily reheated in conventional manners, e.g. in a pot of boiling water. It is known to make slack packages. A method of making such slack packages is shown in European Patent Application 90.300872.0, filed on 1990 Jan. 29 and published 1990 Aug. 8 under No. 0381400. In such application a vertical form, fill and seal machine is disclosed which has a constraint chute below the horizontal sealing jaws, said constraint chute comprising two vertical walls which are adapted to permit a material-filled pouch to travel therebetween, one of said walls being adapted to move away from the other wall under tension and to return to its original position, the degree of tension and the friction of said walls being sufficient to squeeze said pouch therebetween and to permit said pouch to travel therebetween without undue restraint, the plane of both of said walls being substantially perpendicular to the direction of closing of said jaws. Practically, it has been found, such an arrangement is not entirely satisfactory for food-containing pouches, because the pouches do not have sufficient slackness to enable them to be quickly heated when placed in a pot of boiling water. For example, when using such a constraint chute, pouch thicknesses of about 40 to 50 cm are possible. Such pouches may take 40 minutes to heat in boiling water. The present invention is directed to the addition of a simple device for making a slacker package on a vertical form, fill and seal machine. Pouch thicknesses of about 25 cm are achievable, which permit heating of the pouch within about 25 minutes.